Optical fibers have become increasingly important as a medium for transmitting large quantities of information in the form of lightwaves. Each optical fiber comprises a thin solid glass cylinder, known as the core of the fiber, surrounded by an outer layer, known as the cladding, which has a lower refractive index than the core. One way to make such fiber is to make a glass tube, called a substrate tube, and then to vapor-deposit glass that will eventually constitute part of the cladding and the core on the inner surface of the tube as taught, for example, in the U.S. patent of MacChesney, et al. U.S. Pat. No. 4,217,027 granted Aug. 12, 1980, now generally known as the modified chemical vapor deposition (MCVD) technique. After deposition, the entire structure is collapsed to make a preform rod, and glass drawing techniques are used to form a continuous optical fiber from the preform. One way of obtaining the required difference of refractive indices is to dope the glass that will constitute the cladding layer with fluorine, which depresses or reduces its refractive index with respect to that of undoped glass.
For large preforms, an oversized core is deposited in the substrate tube and the collapsed preform rod is inserted into a second tube, called an overclad tube, that is collapsed onto the rod. This is the hybrid or "rod-tube-tube" approach as described, for example, in the U.S. patent of J. W. Baumgart et al., U.S. Pat. No. 4,820,322, granted Apr. 11, 1989. One may likewise wish to dope such overclad tubes with fluorine to depress their refractive indices.
Fluorine-doped glass tubes for use either as substrate tubes or as overclad tubes may be made, first, by depositing glass soot on a mandrel to form a cylindrical porous soot form of glass particulate. Fluorine can be introduced as a dopant by adding a fluorine-containing gas to the reactant stream of the torch during soot deposition, or by adding a fluorine-containing gas to the furnace atmosphere at a temperature below the consolidation temperature. The deposited porous soot cylinder is then consolidated into a glass tube by heating the soot cylinder, which is mounted on a mandrel, in a furnace.
A problem that has been encountered in making fluorine-doped glass tubes is the corrosive damage done to the supporting mandrel by the highly reactive fluorine. This is particularly a problem in the consolidation furnace in which the soot cylinder must be supported by a mandrel for a sufficient period of time at a high temperature to permit sintering or consolidation into a glass cylinder. Other reactive vapors such as chlorine and oxygen also tend to attack the mandrel, which is typically made of graphite. The damage can cause graphite particles to break off and to become entrapped within the consolidated glass substrate, reducing the quality of the glass. The mandrels are often so badly damaged during this process that they cannot be reused. The problem of mandrel damage has been recognized for some time, and it can occur during the soot deposition process as well as during the consolidation process.